1. Field of the Invention
The present invention relates to a heat-developable photosensitive material containing an organic silver salt.
2. Related Background Art
Conventional silver salt photography utilizing a photosensitive silver halide can achieve the excellent sensitivity, gradation, image-stability, etc., and as a recording technique has been most widely used in practice. This method, however, uses a wet process in the processing steps such as developing, fixing and washing, causing many problems on operability and safety, that is, the processing is labourious and time-consuming and chemicals used for the processing may give anxiety for influence on human bodies. Accordingly, it has been sought to provide a photosensitive material utilizing a photosensitive silver halide and capable of being processed by a dry process.
Researchers have been extensively made in relation to such dry processing photography, as disclosed in Japanese Patent Publications No. 43-4921, No. 43-4924, etc. These disclosures are concerned with a technique in which a photosensitive silver halide is used in a catalytic amount and a non-photosensitive organic silver salt is used as an image forming agent. The mechanism by which the organic silver salt acts as an image forming agent can be explained as follows: (1) Silver nuclei are produced from a photosensitive silver halide as a result of exposure, and they form a latent image. (2) The silver nuclei serve as a catalyst, the organic silver salt and the reducing agent cause oxidation-reduction reaction upon heating, and the organic silver salt is reduced to metallic silver, which forms a visible image.
As an example of methods of utilizing such a heat-developable photosensitive material, Japanese Laid-open Patent Application No. 55-50246 discloses a method in which a silver image is used as a mask. As a photosensitive material that can provide images having much better contrast than those with the silver image as a mask, Japanese Laid-open Patent Application No. 3-135564 discloses a photosensitive material capable of forming an image with a good contrast by utilizing light-absorbing properties of a resultant oxidized reducing agent.
In order to accelerate the speed of the development, it is hitherto known to mix a development accelerator such as an alkali agent or an alkali generator into a heat-developable photosensitive material. The development accelerator herein means a substance capable of decreasing the amount of energy required in development when a photosensitive material is exposed to a given amount of exposure light and then developed to obtain a given optical density, and capable of shortening the heating time in the heating step or lowering the heating temperature.
When, however, an alkali agent or alkali generator is used as the development accelerator as conventionally done, there are problems that fogging tends to occur and raw stock stability (the property of retaining the photographic performance of a photosensitive material as it is produced during the storage) is lowered.